St John's wort vs St. John's wort

St John's wort

noun
  • The plant prepared as a medication. 

  • Any of several plants, of the genus Hypericum, especially Hypericum perforatum, that have yellow flowers and traditionally ward off evil. 

St. John's wort

How often have the words St John's wort and St. John's wort occurred in a corpus of books? (source: Google Ngram Viewer )